Including referenced project DLLs in nuget package [.Net Core RC3 *.csproj file]

There is now a more up-to-date workaround described here. Simply add the TargetsForTfmSpecificBuildOutput and Target nodes to your .csproj file as shown below.

<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
    <TargetsForTfmSpecificBuildOutput>$(TargetsForTfmSpecificBuildOutput);CopyProjectReferencesToPackage</TargetsForTfmSpecificBuildOutput>
  </PropertyGroup>
  <Target Name="CopyProjectReferencesToPackage" DependsOnTargets="ResolveReferences">
    <ItemGroup>
      <BuildOutputInPackage Include="@(ReferenceCopyLocalPaths->WithMetadataValue('ReferenceSourceTarget', 'ProjectReference'))" />
    </ItemGroup>
  </Target>
</Project>

Official documentation for this extension point in the pack target can be found here.

You might also then want to add the attribute PrivateAssets="All" to the ProjectReference element to suppress that project from appearing as a NuGet dependency in the generated package, e.g.:

<ProjectReference Include="MyNonNugetDependentProject.csproj" PrivateAssets="All" />